[Nodes]Straight displacement for UV Sphere
I am using cycles with experimental feature set. 
On the uv sphere i am adding adaptive subdivision.

Selecting the area to extrude with less than node


Adding vector displacement


The multiply node is used to change the displacement distance

combine xyz node is used to get the -z direction. The displacement will occurs in this direction.

The normal displacement node displaces along the surface normals, hence you get a conical displacement